Pokkén Tournament’s Lucario has always been Mr. Fundamental: a low-execution, well-rounded character suited to handle any situation adequately. This made him a popular tournament pick, and an unlikely candidate for any major buffs or nerfs in future releases.
There was, perhaps, one tool in his movelist that seemed redundant though: his 2X. Coming in at the speed as his jab (13 frames), this ducking sweep led to nothing if it hit other then a low-damage knockdown. It has shorter range then his other ducking tool (2Y), so aside from situations where your opponent was in Burst Mode, it had little use.
DX solves this by giving 2X a small launch property, allowing you to follow up with 6Y or 2Y for a decent 20% (and potentially more if you hit the wall!). Whether intentional or not, this also combos after Counter Attack stuns, effectively giving Lucario a new optimal path. Mr. Fundamental is fundamentally better, no matter how you look at it.